1. Field of the Invention
The present invention relates to a non-impact dot printer for printing halftone or continuous tone information and the like with small pixels (dots).
2. Description of the Prior Art
Various non-impact recording or printer heads for use as dot printers are known. Examples of such heads include an electrostatic printer head, an LED (light emitting diode) array printer head, ink-jet printer head, thermal printer head, etc. While the invention will find utility with regard to non-impact printers in general, discussion will be made herein with regard to LED printer heads with which the invention is particularly suited.
In U.S. application Ser. No. 07/290,002, the contents of which are incorporated herein by this reference, an LED printer is described in which a series of LED's arranged in a straight line are selectively activatable for brief periods to form dot-like images on a recording surface. In the printer described in this patent, grey scale recording is achieved by having a digital comparator associated with each LED. At one input to the comparator, there is provided the data in the form of a plurality of digital data bits representing an exposure on-time for that LED for that PEL (picture element) period. At a second input to the comparator, an input from an up/down counter is provided that is rapidly changing in accordance with signals emitted from a high speed clock. As the counter output value decreases towards zero at some point in time a match is sensed by the comparator between the two inputs of the comparator (in accordance with its operating criterion). The LED then turns on and remains on for a very brief duration until a subsequent sensing of a match by the comparator occurs during a count-up phase of the counter. In the above noted application "exposure space" is enhanced by providing a programmable clock whose periodicity changes within the counting cycle. The gradations of exposure provided are more realistically related to human visual perception. The above application also notes that the data may be adjusted to provide also for exposure balancing of the LED's. This is desirable where non-uniformity in illumination from LED to LED on the printer head may be expected.
A problem with the above is that it does not provide for uniformity correction to the extent that may sometimes be required for particularly high quality continuous tone or halftone printing.
It is an object of this invention, therefore, to overcome the deficiencies of this prior art.